This invention relates to an enhanced oil recovery process and more particularly to an oil recovery process for use in subterranean petroleum formations containing two or more zones which differ from one another in permeability.
It is well known that only a small fraction of the petroleum originally present in a subterranean petroleum formation is recoverable by primary production i.e. by allowing the oil to flow to the surface of the earth as a consequence of naturally occurring energy forces. Secondary waterflooding of many reservoirs recovers only a small portion of the remaining oil because of the presence within the reservoir of a number of strata of widely varying permeability.
In general, secondary recovery processes comprise injecting water into the formation of displace petroleum through the formation and finally to the surface of the earth. The total oil recovery efficiency of the process is frequently poor because when the fluid is injected into a heterogeneous reservoir, the fluid passes primarily through the most permeable zones and little or none of the fluid passes through the lower permeability zones. If only two strata are present differing by only a factor of two in their permeability nearly all flow will begin through the more permeable stratum. This flow of water through the more permeable stratum removes some oil which will result in the more permeable stratum becoming even more permeable, thus aggravating the problem.
Water does not displace oil effectively since water and oil are immiscible and the interfacial tension between water and oil is quite high. Thus, surfactants are generally added to the flood water to reduce the interfacial tension between the injected aqueous fluid and the formation petroleum thereby increasing the microscopic displacement efficiency of the injected aqueous fluid. However, even if the surface tension between the injected aqueous fluid and the petroleum present in the subterranean reservoir can be reduced by incorporating surfactants into the injected fluid, the total oil recovery efficiency of the process is still frequently poor.
Several U.S. patents disclose the concept of injecting various surfactants to selectively plug the more permeable strata so that the waterflooding efficiency will be improved in the less permeable strata. In this regard, Schievelbein et al. in a series of U.S. patents, namely U.S. Pat. No. 4,160,480, U.S. Pat. No. 4,161,982, U.S. Pat. No. 4,161,983, U.S. Pat. No. 4,184,549, U.S. Pat. No. 4,192,382 and U.S. Pat. No. 4,194,563, disclose the injection of various surfactants and mixtures of surfactants which form coarse macroemulsions to reduce permeability.
Hess et al., "Chemical Method for Formation Plugging," J. Pet. Tech. 551, May 1971 disclose a process for injecting an acidic solution followed by furfuryl alcohol resin into a petroleum formation which flows preferentially into highly permeable zones and then forms a plug. The process is based on the acid-catalyzed polymerization of furfuryl alcohol resins. It is applied by injecting an acidic solution into the interval to be plugged, followed by the resin solution. The two solutions mix in the formation to start a rapid, vigorous exothermic reaction forming a hard solid that fills the pore space.
Terry et al., "Correlation of Gelation Times for Polymer Agents," Soc. Pet. Eng. J., 229, April, 1981 disclose the use of gelled polymers (such as polyacrylamides) to redirect or modify reservoir fluid movement in the vicinity of injection or production wells for the purpose of improving water/oil ratios.
Other processes involve the use of hydrophilic polymers including partially hydrolyzed polyacrylamide, copolymers of acrylamide and acrylic acid or water soluble acrylates, polysaccharides, etc. Unfortunately, the fluids employing these hydrophilic polymers are substantially more viscous than water at the time of injection and so injection into the zones is difficult and there is little likelihood that they will invade the same zones as would be invaded by water or another aqueous fluid having about the same viscosity as water.
Accordingly, the effectiveness of the above-described processes has been restricted to reducing the permeability of only the most permeable flow channels in a zone and is furthermore usually restricted only to near wellbore zone of the formation, e.g. that portion of the most permeable zone in a formation immediately adjacent to the injection well, because of the difficulty of injecting viscous fluids through great portions of the formation.
Other methods which can overcome the problem of differential flow in subterranean petroleum formations and which will allow some tertiary oil recovery from the more permeable zones are of course desirable. There is a substantial need for a method of treating heterogeneous formations to reduce the permeability of the very high permeability zones to force subsequently injected oil displacing fluids to pass into those zones which were originally of lower permeability and thus were not invaded by the first injected fluids.